1. Field of the Invention
The present invention relates to a sensor capable of detecting zinc oxide nanowires in water with high selectivity and sensitivity and a method for the detection of zinc oxide nanowires in water using the sensor.
2. Description of the Related Art
Nanomaterials have recently attracted a considerable attention due to their physical, chemical, optical, and electrical properties different from the typical properties of their bulk phase (Moezzi et al., 2012). Thus, the use of such nanomaterials has grown rapidly for commercial purposes as well as research purposes. Various commercial products, including sunscreens, tennis rackets, solid lubricants, and detergents, use nanomaterials to enhance their performance. Among various types of nanomaterials, zinc oxide nanowires (ZnONWs) have received great interest due to their unique piezoelectric and semiconducting properties (Chen et al., 2011; Hempen and Karst, 2006; Huang et al., 2006; Nasr et al., 2013; Riaz et al., 2011; Wang et al., 2008; Williams and Kamat, 2009) and have been explored for various applications, including piezoelectric devices, energy harvesting devices, self-powered nanosensors, and biomedical devices (Agrawal and Espinosa, 2011; Lin et al., 2012; Rasmussen et al., 2010; Wang and Song, 2006).
The toxicity of zinc oxide nanowires to humans and the environment needs to be analyzed before their use in industrial and commercial products. Thus, studies on the toxicity of zinc oxide have recently been reported (Nel et al., 2006). Since zinc oxide was declared as a toxic substance by the Organization for Economic Cooperation and Development (OECD), its potential toxicity has been a concern.
Specifically, as the size of zinc oxide nanowires decreases to the nanometer scale, the surface area per unit volume increases extensively and chemical reactions occur to cause rapid production of reactive oxygen species (ROS) in organisms (George et al., 2009). Eventually, the rapid production of ROS causes damage to mitochondria, cell membrane, and nuclear DNA, leading to the malfunction of enzymes or the death of cells (Ryter et al., 2007). Further, when zinc oxide nanowires enter cells, a degradation process occurs to generate zinc ions (Zn2+) due to the weakly acidic environment. The zinc ions tend to cause toxic phenomena, which are potentially harmful to the cells. The critical concentration of zinc oxide nanowires for human monocyte macrophages was reported to be 10 μg/mL or less in a weakly acidic solution (Müller et al., 2010). Generally, a concentration of 25 μg/mL of zinc oxide is considered as the effective toxicity in neutral solutions (George et al., 2009; Xia et al., 2008).
As applications based on zinc oxide nanowires develop, human exposure to zinc oxide nanowires increases, which is considered as a potential hazard to human health. Due to their very small size, an increased amount of zinc oxide nanowires will be discharged in the form of industrial waste to the water system. When exposed to the environment, zinc oxide nanowires would be accumulated in the human body along the food chain, which will adversely affect human health later on. Thus, there is an increasing need to monitor the concentration of zinc oxide nanowires.
Numerous approaches to the detection of toxic nanomaterials in water have been proposed, for example, by atomic absorption spectroscopy (M. Ghaedi, F. Ahmadi and A. Shokrollahi, J. Hazard. Mater., 2007, 142, 272-278), fluorometry based optical methods (Y. Wen, F. Xing, S. He, S. Song, L. Wang, Y. Long, D. Li and C. Fan, Chem. Commun., 2010, 46, 2596-2598), surface plasmon resonance (T. Kang, S. Hong, J. Moon, S. Oh and J. Yi, Chem. Commun., 2005, 3721-3723), surface-enhanced Raman scattering (J. Yin, T. Wu, J. Song, Q. Zhang, S. Liu, R. Xu and H. Duan, Chem. Mater., 2011, 23, 4756-4764), electrochemical methods (Z. Lin, X. Li and H. B. Kraatz, Anal. Chem., 2011, 83, 6896-6901), and methods using resonators (J. Park, W. Choi, K. Jang and S. Na, Biosens. Bioelectron., 2013, 41, 471-476). These approaches are useful for selective detection of many nanomaterials but, to our knowledge, no successful methods for selective detection of zinc oxide nanowires in water have been reported to date.